Graphene is a two-dimensional crystal comprised of carbon atoms and has drawn attention since it was found in 2004. Graphene has excellent electric, thermal, optical, and mechanical properties and is expected to be widely applicable in the fields of battery materials, energy storage materials, electronic devices, composite materials, etc. Two professors of the University of Manchester, UK, were awarded the Nobel Prize in Physics in 2010 for the results of their studies on graphene.
As a production method of graphene, there are a mechanical exfoliation method, a chemical vapor deposition method, an epitaxial crystal growth method, a redox method, etc. Among them, the former three methods require complicated processes and have low productivity and are thus difficult for mass production. On the other hand, the redox method has a feature of easiness of mass productivity and chemical modification and has thus drawn attention.
A redox method proposed presently is for producing graphene by reducing graphene oxide with a heat reduction method or a technique using reducing agents such as hydrazines or other organic substances.
The heat reduction method involves heat reduction of graphene oxide at a high temperature of 1050° C. to produce graphene (Patent Document 1). However, the production method requires the high temperature reaction and thus requires facilities at a high cost.
A known example is a method of producing graphene by reducing graphene oxide in a reaction at 100° C. for 24 hours in water, using a hydrazine hydrate as a reducing agent (Non-Patent Document 1). However, reducing agents of hydrazines are deleterious substances and unsuitable for industrially practical use and take a long time for the reaction. Consequently, it is required to seek a simple and highly efficient reduction method with a low poisonous property.
On the other hand, another known example is a method of producing graphene by reducing graphene oxide using an organic substance containing an amino group such as urea as a reducing agent (Patent Document 2). This reaction system is low poisonous, however, urea is an organic substance with a low reduction property and the reduction reaction of graphene oxide is insufficient.
Further, with respect to the properties of the obtained graphene, since the graphene obtained by the heat reduction (Non-Patent Document 2) has high crystallinity, lamination of graphene in the layer direction is easy to occur and agglomeration tends to be caused easily. Further, the graphene obtained by hydrazine reduction has a structure replaced with nitrogen, resulting in occurrence of a problem of a decrease in conductivity.
Unlike the above-mentioned methods, a technique of producing graphene without undergoing graphene oxide has also been proposed. For example, there is a technique of producing graphene by exfoliating interlayer of a graphite intercalation compound by quickly heating the graphite intercalation compound (Non-Patent Document 3). However, the graphene obtained by this technique is graphene which is produced by a technique without undergoing graphene oxide and has a less amount of functional groups to be contained and is not dispersed in a solvent or a resin, so that agglomeration tends to be caused easily.
Further, a technique for producing graphene on a substrate by CVD is also known; however the obtained graphene is not a powder but in the form of a film and the graphene produced by the technique is supposed to have a considerably high agglomeration property even if the grapheme is formed into a powder since it scarcely contains oxygen atoms.